In vivo characterization of histamine H1- and H2-receptors in the rat stomach microcirculation.

1 Using a video microscope system, an in vivo microcirculation preparation was designed to characterize histamine receptors in the rat stomach submucosal arterioles (diameter 50 microns). 2 Each in vivo stomach microcirculation preparation received topically applied multiple concentrations of either the selective histamine H1- or H2-receptor agonist and antagonist and the respective responses (changes of the arteriolar diameter) were used to construct the concentration-response curves. 3 Results showed that both 2-thiazolylethylamine and impromidine, the respective selective H1- and H2-receptor agonists, produced concentration-dependent vasodilator responses in the in vivo stomach submucosal arterioles. These vasodilator responses were competitively and selectively blocked by the respective H1-receptor antagonist mepyramine and the H2-receptor antagonist cimetidine. 4 Data from each in vivo preparation were examined separately to yield a Schild plot and a Hill plot, from which the in vivo estimates of the pA2 value, the slope of the Schild plot, and the Hill coefficient were obtained. 5 The estimated pA2 values for mepyramine (9.60 +/- 0.033, mean +/- s.e. mean) and cimetidine (5.98 +/- 0.037) conformed to similar values found in other tissues, showing that the rat stomach microvascular H1- and H2-receptors are of the same nature as similar receptors elsewhere. 6 Both the Hill and the Schild plots yielded regressions with unity slopes, indicating that the agonist-receptor and the antagonist-receptor reactions followed a simple one-to-one stoichiometry. 7 The findings in the present study are discussed and compared with those from other in vitro tissue preparations; it appears that the present in vivo technique is a satisfactory system for characterizing receptors in the vascular smooth muscle of the microcirculation.